Natural tissues obtained from mammalian sources have been used for allogenic and xenogeneic grafting for many years. For example, certain cardiovascular tissues (e.g. segments of blood vessel, heart valves) have been harvested from human or other mammalian sources and subsequently surgically implanted in the human body.
Many of the biological tissues which one desires to harvest and prepare for subsequent surgical implantation contain substantial amounts of connective tissue. The function of the connective tissue is to provide a supportive frame work within which other functional cell types (e.g. muscle fibers) are disposed. Connective tissues are largely formed of insoluble proteins collagen and elastin. Collagen and elastin exist in the form of insoluble fibers. Such insoluble fibrils are arranged within a continuous matrix called the ground substance. The flexibility and other characteristics of the connective tissue depend largely on the proportions of collagen and elastin contained within such tissue, and the structure and configuration of the collagen/elastic fiber network thereof.
As illustrated in FIGS. 5a-5c herein, each collagen molecule consists of three polypeptide chains intertwined in a coiled helical conformation. The individual amino acid constituents of each polypeptide chain are connected, by hydrogen bonds, to individual amino acids of an adjacent polypeptide chain, thereby holding the individual polypeptide chains in the triple helical conformation shown in FIG. 5c.
The current methods of preserving and preparing collagenous, biological tissues for subsequent surgical implantation include the step of "fixing" the collagen network by exposing the tissue to one or more chemical compounds capable of cross-linking the collagen molecules within the tissue. Both intermolecular (FIG. 5b) and intramolecular (FIG. 5c) cross-linkages may be formed by the currently known fixative compounds.
Chemical compounds which are known to cross-link collagen include formaldehyde, glutaraldehyde, dialdehyde starch, hexamethylene diisocyanate and certain polyepoxy compounds including glycol diglycidyl ether, polyol polyglycidyl ether and dicarboxylic acid diglycidylester. Three (3) specific water soluble polyepoxy compounds which may be used as collagen cross-linking agents are shown below: